The present invention relates to the technique thickness-reduction rolling of hot thin plate material, and more particularly to a technique of this kind which effects the thickness-reduction rolling of the thin plate material while suppressing a temperature drop of the thin plate material by effectively removing scale formed on the surface of the hot thin plate material.
A hot rolled thin plate has conventionally been manufactured as disclosed in Japanese Patent Application Laid-Open Publication No. 143949/77 by the method which comprises: rolling a slab having a thickness of 200 to 300 mm by a rougher disposed downstream of a widthwise rolling mill which rolls the slab in the direction Of width thereof, thereby obtaining a plate having a thickness of 20 to 40 mm; thereafter allowing the plate to pass through a descaling device jetting a high pressure water in the order of 150 kg/cm.sup.2 so as to remove scale on the surface of the plate; and finish hot rolling the resulting plate thereby obtaining a thin plate product having a desired thickness.
With this method, however, since the plate material to be rolled is thin, the descaling by a high pressure water performed before the finish rolling causes a large temperature drop of the plate material. Thus, this method has a problem that it necessitates reheating of the plate material to be rolled and thus calls for consumption of an excessive amount of energy.
In particular, attempts have recently been made to continuously manufacture a thin plate ingot having a thickness of 20 to 40 mm by a continuous casting machine as disclosed in Japanese Utility Model Application Laid-Open Publication No. 56145/85, and when hot rolling the thin plate ingot by utilizing a casting sensible heat and thus without reheating, a temperature drop of the thin plate ingot during the descaling step becomes a problem. Therefore, it has become very important to develop a descaling technique which does not cause a large temperature drop.
That is, at an inlet side of a hot finishing mill it is necessary that the temperature of a material to be rolled is in the order of 1000.degree. C.; but since the temperature of a thin plate ingot continuously obtained by the continuous casting has already reached a temperature of about 1000.degree. C. before being rolled, it is a fatal problem that the temperature of the ingot is further lowered by high pressure water descaling. Since, in, general the temperature of a material to be rolled is lowered by about 80.degree. to 100.degree. C. by the high pressure water descaling, it is desirable to develop a descaling method which can suppress this temperature drop to a minimum. As an example of a descaling method which causes only a small temperature drop, a method in which the scale on the surface of a material to be rolled is mechanically removed by means of brushes in addition to bending roll has been known from Japanese Utility Model Application Laid-Open Publication No. 167421/77, but this method is not practical because the brushes are excessively worn, so that a further new descaling method is desired.
As disclosed in the above-mentioned Japanese Utility Model Application Laid-Open Publication No. 167421/77, a method is well known in which cracks are formed in the scale by cold bending the plate thereby enhancing an effectiveness for the pickling which is a post-treatment for removing the scale. However, in case where only this bending work is effected a series of cracks, each of which extends widthwise, are formed in the scale only in the plate's longitudinal direction, so that an amount of the scale exfoliated from the parent material is very small.